1. Field of the Invention
This invention relates to line drivers and, more particularly, to techniques for reducing ringing and power losses in line driving systems.
2. Description of Related Art
Line reflection decreases the noise margin in high-speed digital circuits, especially line reflections induced by signal buffers driving off-chip loads.
The signal buffer acts as a line driver. After the driver causes a signal transition, the transition travels from the source (near) end of the transmission line to the load (far) end. Upon reaching the end, the signal transition is usually reflected at the far end and travels back toward the source. The reflected signal is then usually again reflected upon reaching the source back towards the load. This process continues until the cumulated losses cause the reflection to die out. The resulting voltage waveform seen at either end of the transmission line is typically an exponentially-damped oscillation, often referred to as “ringing.”
Ringing often creates problems. It often causes the voltage on the transmission line to exceed allowable or safe levels. Therefore, circuitry connected to the line must be designed to accommodate higher voltage levels than are actually needed. The ringing can also be erroneously interpreted as a change in the state of the data on the line.
In the past, efforts have been made to match the impedance of the driver to the transmission line and/or the impedance of the transmission line to the load. If the impedance at one end or the other is perfectly matched, there would normally be no reflection.
A simple approach used to match the impedance between the transmission line and the load is to connect a resistance at the end of the transmission line to ground. This approach, however, causes additional power to be dissipated in the resistance that is added. This is undesirable in low-power applications, such as in a VLSI pin driver used for fast chip-to-chip communication.
A simple approach for matching the impedance between the driver and the transmission line is to insert a resistance in series between the output of the driver and the input of the transmission line. Again, however, the addition of such a resistance increases power dissipation.
Another problem with line driving systems is the dissipation of power that occurs during transitions of the signal. This is particularly true when the load includes a substantial capacitive reactance, such as in a VLSI pin driver used for fast chip-to-chip communication.
In short, there is a need for a driver that drives a transmission line connected to a load which substantially reduces ringing without wasting power and which, preferably, reduces the energy that is dissipated during operation.